1. Field of the Invention
This invention relates generally to tube and fin heat exchangers, and in particular, to manufacturing processes and equipment for producing tube and fin heat exchangers made with aluminum tubing, such as for HVAC systems.
2. Description of the Prior Art
As illustrated in FIG. 1, a typical tube and fin heat exchanger (10) consists of a stack of generally planar aluminum fins (12) sandwiched between a top end plate (14) and a bottom end plate (16). The terms “top” and “bottom” used for designating heat exchanger end plates are derived based on the heat exchanger orientation during expansion in a vertical hairpin expander press, as described below. The “top” and “bottom” designations are not necessarily indicative of the heat exchanger orientation in any particular installation.
The fins (12) have a number of collared holes (18) formed therethrough, and the top and bottom end plates (14, 16) have corresponding holes (20) formed therethrough. When the fins (12) and end plates (14, 16) are stacked, the holes (18, 20) are in axial alignment for receiving a number of U-shaped hairpin tubes (“hairpins”) (22) through the stack. Hairpins (22) are formed by bending lengths of small tubes 180 degrees around a small diameter mandrel. The hairpin tubes (22) are fed, or laced, through the loosely-stacked assembly of fins from the bottom end plate (16) so that the open ends (26) of the hairpin tubes (22) extend beyond the top end plate (14). The top end plate (14) is slipped over the open ends (26) of the hairpins (22), and the hairpins (22) are either brazed to the fins, or are mechanically expanded from within using a hairpin expander to create a mechanical interference fit with the fins (12). U.S. Pat. No. 4,645,119 issued to Haranaki, et al. describes a process in which hairpins (22) are brazed to the fins (12). Co-pending U.S. patent application Ser. No. 12/139,379 filed on Jun. 13, 2008 in the name of Dees, et al., describes a typical manufacturing process for making heat exchangers with aluminum fins and copper hairpin tubing in which the hairpins are expanded into interference engagement with the fins. The distance that the hairpin ends (26) extend beyond the top end plate (14) is referred to as the hairpin “stickup” distance. The stickup distance is typically about ½ inch. Finally, return bend fittings (24) are brazed to the open ends (26) of the hairpin tubes (22) to create a serpentine fluid circuit through the stack of fins (12).
Heat exchangers can be made of various metals. The most prevalent materials used are aluminum for fins (12) and copper for hairpin tubes (22). However, due to corrosion concerns and also in part due to the rising price of copper, there is desire among HVAC manufacturers to transition a greater number of production lines for the exclusive manufacture of tube and fin heat exchangers with both aluminum fins and aluminum tubing.
FIG. 2 is a flow chart diagram that describes a typical manufacturing process of prior art used to mass produce aluminum tube and fin heat exchangers using the hairpin expansion process. Referring to both FIGS. 1 and 2, as shown in step (50), fins (12) are formed by a stamping process in a fin press, such as that produced by Burr Oak Tool, Inc. of Sturgis, Mich. Aluminum fin stock is delivered to a press in a roll of sheet metal. Fin stock is paid out from an uncoiler, lubricated, then fed through the fin press, where a die draws, details, punches collared holes, and cuts fins to a desired length and width. As the process for producing fins is well known to a routineer in the art, it is not discussed further herein. Fins (12) are stacked and staged for the lacing process as depicted by element (58) in FIG. 2.
As shown by step (52) in FIG. 2, the heat exchanger top and bottom end plates (14, 16) are manufactured in a stamping process that is independent of the fin stamping process (50). The end plates are typically made of a fairly stiff sheet metal. The end plates (14, 16) may also each include bends that form a channel or similar profile to provide strength and rigidity. Holes (20), which align with the collared holes (18) of the fins (12), are punched through the end plates by a press and die.
The hairpin tubes (22) are manufactured in process step (54). Hairpins are typically formed in a hairpin bender machine, such as a vertical bend hairpin bender manufactured by Burr Oak Tool, Inc. of Sturgis, Mich. Depending on the outer diameter of the stock tubing, commonly up to six lines of tubing are typically processed simultaneously in a single hairpin bender machine. As the process for bending hairpin tubes is well known to a routineer in the art, it is not discussed further herein.
In step (56), return bend fittings (24) are formed by a return bender machine or a return elbow bender machine such as those manufactured by Burr Oak Tool, Inc. of Sturgis, Mich. Return bender machines automatically bend and cut stock tubing to form the return bend fittings (24). As illustrated in FIGS. 3 and 4, the ends (25) of prior art return bend fittings (24) are cut square. Cross-over fittings are also manufactured as is known in the art. Details of these process steps are well known to routineers in the art and are thus not discussed further herein.
Referring back to step (58) of FIG. 2, the lacing process is that process in which the bottom end plate (16), stacks of fins (12), the top end plate (14), and the hairpins (22) are assembled together, typically by hand. Fins are stacked on a lacing table to form a contiguous slab of fins. The heat exchanger bottom end plate (16) is added to one end of the slab, and the end plate 14 is added to the other end of the slab. Hairpins (22) are typically hand-laced through the bottom end plate (16), the slab of fins (12), and the top end plate (14), one at a time by an operator who manually finesses them.
After lacing step (58), the heat exchanger assembly consists of stacks of fins (12) and a bottom end plate (16), which are loosely held together by hairpins (22) passing transversely through the assembly. In a corresponding manufacturing process for tube and fin heat exchangers that have copper hairpin tubes instead of aluminum hairpin tubes, the assembly would next be expanded within the hairpin expander in order to form tight metal-to-metal interfaces between the tubes and the fins of the heat exchanger. However, because of the abrasive nature of aluminum material, a processing oil is typically first injected into the interior of aluminum hairpins to lubricate the hairpin expansion bullets during hairpin expansion. Without a heavy layer of oil lubricant, the hairpin expander bullets tend to become galled with aluminum. Thus, as shown in step (60), an ordinary metalworking lubricant, for example, mineral oil, is injected into the hairpins (22).
As described with reference to step (62) of FIG. 2, the laced and oiled heat exchanger assembly is placed within a hairpin expander machine, such as a vertical hairpin expander available from Burr Oak Tool, Inc. of Sturgis, Mich. The top end plate (14) is slipped over the open ends (26) of the hairpins (22). The hairpin expander has bullets located at the ends of long rods for passing through the open ends of the hairpins. Multiple bullets and rods, two for each hairpin, are typically provided for simultaneously expanding all of the hairpins. Each bullet is sized to have an outer diameter larger than the inner diameter of the hairpin tubes. The expander has a hydraulic ram, that drives rods and presses the bullets into the hairpins, the bullets expanding the hairpins into a tight, interference-fit engagement with the fins (12). As shown in FIGS. 3 and 4, the hairpin expander also flares the ends (26) of hairpins (22) to create a socket (27) for receiving return bend fittings (24) or cross-over fittings.
The return bend fittings (24) are usually connected to the ends (26) of hairpins (22) by autobrazing, in which flux and filler metal (typically applied as a cladding) are prepositioned at the braze joints and the assembly is passed through an oven or furnace at a temperature that causes the filler metal to melt and flow to create a solid joint without any melting of the base metal. Brazing requires the joint surfaces to be particularly clean and free of non-metallic surface particulates. Therefore, after the expansion process, the assembly is typically washed in a hot aqueous solvent bath and/or flushed with an aqueous solvent to remove the lubricating oil that was applied for the expansion process (62). A typical aqueous washer, such as that available from Seco/Warwick Corp. of Meadville, Pa., is a multi-stage washing unit including automatic pre-wash, wash, rinse and dry chambers through which the heat exchangers are conveyed. The washer removes processing oils, dirt and aluminum fines from the heat exchanger assemblies. This aqueous washing/flushing process is costly, because the solvent becomes contaminated, requiring disposal in compliance with strict environmental regulations, and because a significant amount of energy is required to heat and maintain the solvent bath at elevated temperatures. Moreover, aqueous washers are high capital-cost items.
As an alternative to the aqueous washing cycle, a thermal degreasing oven may be used to vaporize light evaporative processing oils from the heat exchanger surfaces. Thermal degreasing ovens, such as those available from Seco/Warwick Corp. of Meadville, Pa., typically operate at 250-300° C. Heat exchangers are passed through the oven on a conveyor belt. Thermal degreasing ovens will only remove processing oils, not aluminum particulate.
After the cleaning process (64), the return bend fittings (24) and cross-over fittings are hand-assembled with prefluxed brazing rings to the open ends (26) of the hairpins (22) at step (66) of FIG. 2. Braze rings typically used in prior art processes are 88 percent aluminum and 12 percent silicon with Nocolok flux. Referring to FIGS. 3 and 4, the profile of the end of return bend fitting (24), which is cut perpendicular to the axis of the tube, does not mirror the profile of the tapered part of the hairpin socket (27). On occasion, this profile mismatch can result in misassembly of the return bends and concomitant poor braze joints in socket (27).
FIG. 5 illustrates a section of a typical gas-fired open flame furnace (80) used for autobrazing copper return bends (24) to copper hairpins (22). The furnace (80) has two gas headers (82) from which burner assemblies (84, 86) extend. Heat exchanger assemblies pass longitudinally parallel to and midway between the gas headers (82) through the furnace (80) by way of a conveyor system (not shown). Each burner assembly terminates with an orifice (88) that is dimensioned to produce a narrow, sharp “pencil-point” flame. The burner assemblies (84, 86) are positioned to locate the flames and concentrate the heat directly at the braze joints as the heat exchanger assemblies pass by. For single or double row heat exchangers, only horizontal burners 84 are required to direct the flames at the return bend joints. When heat exchangers have 3 or 4 rows, such as shown in FIG. 5, angled burners 86 are required to direct the flame at inner return bend joints.
However, the open flame brazing furnace of FIG. 5 is not used to braze aluminum return bends to aluminum hairpins. Because aluminum is a highly reactive metal, it spontaneously oxidizes in the presence of the earth's atmosphere, forming a tenacious aluminum oxide layer that reduces wettability and inhibits the flow of the filler material at the braze joint. Therefore, autobrazing is performed in either a vacuum oven or a controlled-atmosphere oven. Non-corrosive fluxes such as Nocolok fluxes, which become sufficiently activated at the higher temperatures of the braze oven, are applied to strip the oxide layer to allow a wetted braze joint in the absence of oxygen.
Controlled atmospheric brazing (CAB) has superseded vacuum brazing as the preferred process for manufacturing tube and fin heat exchangers, because a CAB furnace, such as that available from Seco/Warwick Corp. of Meadville, Pa., is generally less expensive to purchase, requires less maintenance, and has a higher throughput than a vacuum furnace. A CAB process for use with aluminum heat exchangers is described in U.S. Pat. No. 5,771,962 issued to Evans, et al. or U.S. Pat. No. 6,512,205 issued to Evans. As depicted in step (68) of FIG. 2, the heat exchanger assembly is run through a CAB furnace, wherein the joints are brazed.
Although CAB is generally preferred over vacuum brazing, a CAB furnace is still an expensive piece of equipment, which requires regular maintenance, and which is characterized by a low throughput. For example, a typical CAB furnace may cost in excess of $4 million. It is desirable, therefore, to provide a process and system that results in a more efficient manufacturing of all aluminum tube and fin heat exchangers at lower cost by eliminating the need for controlled atmospheric brazing and for aqueous washing of tube and fin heat exchangers.